In hybrid vehicles, an electric motor or a motor-generator (as will be simply called a “motor” hereinafter) is used as a prime mover together with an engine, and an operating mode can be selected from a various kinds of modes. For example, Japanese Patent Laid-Open No. 2008-265598 describes a two-motor type hybrid drive unit having a motor/generator for controlling an engine speed and a motor activated by a power generated by the motor-generator. According to the teachings of Japanese Patent Laid-Open No. 2008-265598, the engine and the motor/generator are connected to a power distribution device having three rotary elements, and the motor is connected to an output element of the power distribution device to deliver an electric power thereto from the motor-generator. the output element is also connected to driving wheels through a gear train, and an output shaft of the engine is halted by a halting means.
That is, the output element of the power distribution device is subjected to a negative torque resulting from rotating the driving wheel. In this situation, the engine speed is changed in accordance with a speed of the motor-generator by operating the engine while activating the motor-generator to generate power. Consequently, the hybrid vehicle is allowed to be propelled in an optimally fuel efficient manner. In this situation, the electric power generated by the motor-generator is delivered to the motor, and a power generated by the motor is delivered to the driving wheels through the output element. That is, the hybrid vehicle is propelled under engine mode (or hybrid mode). According to the teachings of Japanese Patent Laid-Open No. 2008-265598, the hybrid vehicle is also allowed to be propelled under motor mode. In the motor mode, the output shaft of the engine is halted by the halting means so that the power distribution device may serve as a speed change device. Consequently, an input element of the power distribution device is halted so that torque of the motor/generator can be outputted from the output element while being increased or decreased in accordance with a gear ratio of the power distribution device. As a result, the driving wheels are rotated by driving forces of the motor and the motor-generator.
Thus, according to the teachings of Japanese Patent Laid-Open No. 2008-265598, operating mode of the hybrid vehicle can be selected from the engine mode and the motor mode, and hence fuel efficiency can be improved by efficiently activating the motor by a regenerative energy. That is, a prime move can be selected in accordance with a required driving force based on an opening degree of an accelerator changed by a driver's operation. In addition, if the required driving force is small, the vehicle may also be propelled by any one of the motors to further improve the energy efficiency.
The operating mode of the hybrid vehicle is selected from those two or three modes in accordance with an opening degree of the accelerator and a vehicle speed. For example, if the vehicle is propelled at a predetermined speed by opening the accelerator widely, the engine mode is selected. By contrast, if the opening degree of the accelerator is small, the motor mode is selected to propel the vehicle by both motors, and if the opening degree of the accelerator is further small, the vehicle is propelled by any one of the motors. That is, the operating mode is shifted in response to a change in the opening degree of the accelerator or a required driving force. Therefore, when the accelerator pedal is depressed or returned abruptly, the operating mode is shifted suddenly. In this case, in the hybrid vehicle taught by Japanese Patent Laid-Open No. 2008-265598, the operating mode can be selected from the above-explained three modes. However, if the operating mode is shifted only in response to a change in the opening degree of the accelerator, the operating mode may be shifted immediately across two modes. In those operating modes, an interim mode is merely a temporary mode during a shifting operation, however, operating states of the motors and engagement states have to be changed even when shifting the operating mode to such an interim mode. For this reason, a longer time is required to shift the operating mode, and a driver may feel discomfort resulting from a change in the driving force caused by the shifting operation.